Beam in sports venue back to front (4)
I believe the answer is:
grin
'beam' is the definition.
(I've seen this before)
'in sports venue back to front' is the wordplay.
I cannot really see how this works, but
'in' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for grin that I've seen before include "A broad smile showing the teeth" , "Smirk" , "Ring (anag.)" , "Accept with good grace - . . . . and bear it" , "Smile - all that could be seen of the vanishing Cheshire Cat" .)